P0740

[Shadrach]

I just got back from a trip thru the mountain of Alberta and had my CEL come on twice. Both time is is P0740 and P0700. The first time was just into the trip and then just before I got home again. I can't "make" the light come on by doing any particular thing. I appears to be random. The 68 RFE has an H & S program in it and an after market TC. I have checked the plugs at the front and rear on the trans and can't "see" anything obvious. Any ideas?
David

 

[transe]

P0740 (TCC Out of Range) means the Torque Converter Clutch is not engaging properly (it is slipping when it should be locked). The problem is typically in the converter (bad TCC) or in the pump (sticking valve or worn pump). Have to remove the trans to get at either one, unfortunately.

If yours is only intermittent, you may be able to just ignore it for the time being. When the P0740 sets, it will disable the TCC. When the TCC is disabled, you will generate more heat in the converter (than usual), therefore the trans will run hotter and may eventually (on a long trip) overheat. So if the CEL comes on, restart the truck (whenever it's convenient) so you'll regain TCC operation. Other than that, P0740 will not cause any issues (no limp-in, missing gears, loss of power, etc.), so if you can reset it and it then works OK, I would do that until it starts happening more frequently. Once it gets too annoying, then you'll probably need to replace the pump and converter. But it's not an emergency.

Oh, and there is one other downside to P0740: your fuel economy goes down since the TCC is not engaging. So that's one more reason to reset it each time it sets.

 

[realfine409]

TE, I have just experienced the same issue. Presently in TX, working our way back home (NC). Noticed engine speed increasing when going up small grade, (against 25 mph headwind), but vehicle speed remained steady (65mph,cruise on, 12k 5er). Indeed, got both p0700 and p0740 codes on my reader. Is it the TC that goes bad or the TC solenoid?when the TC slips does that sheer the fluid therefore generating heat? If I manually kick it down to 5th prior to the slippage, would it get me back home? If I drop the transmission at home and find a decent shop, what else should I consider replacing after 1800 miles of sporadic shifting? Truck is bone stock, any transmission upgrades while apart?

 

[transe]

You should be able to drive it OK with no damage, as long as you stop and "reset" it whenever you notice the TCC is no longer locking up. P0740 will completely disable the TCC, and if you drive it for an extended period with TCC unlocked you will overheat it.

Sounds like a "weak" TCC (that only slips at high torque levels). So downshifting (as you did) is a good idea, as this will reduce the amount of torque required to maintain speed. I think if you just manually downshift it on grades you should be able to drive it OK.

The solenoid is not usually the problem (if it were, you would likely set some other fault codes too). You likely have either a bad TCC (bad converter), or a worn TCC control valve in the pump. A worn valve bore causes a higher-than-usual amount of pressure needed to hold the TCC (which will also set the P0740 fault). So the typical fix is to replace both the pump assy and the converter. That is what I would recommend for a 2007-2008 truck with P0740. While doing that, I would also pull the main valve body and check for wear in the Solenoid Switch Valve (SSV) bore, which is a very common problem in 2007-2009 trucks at high mileage. If the bore is worn, replace the main VB as well. And while the pump is out, you might want to have them also pull the input clutch assy (the next big "chunk" of parts behind the pump) and check your UD, OD, and Reverse clutch packs (which are inside this assy) for any heat / wear. You can then replace those if there are any issues.